Relief valves, from their beginning with a simple ball and spring, have had widely varying characteristics as well as problems, many of which were ignored in the past. Today, however, due to the requirements for increased machine productivity and improved efficiency demands, the performance characteristics of relief valves have become more important and the design criteria more strict.
In the past, the ability of a relief valve to shut off after it has opened has not been a major concern, so consequently many have had very high hysteresis with a closing pressure substantially less than its opening pressure. Today in many mobile hydraulic applications, it is required that the valve close within a given band of pressure from the opening curve, as for example 100 PSI.
The reason relief valves close at a lower pressure than they open basically relates to mechanical friction in the valve. Designers today utilize low friction seals as one method to decrease the hysteresis of the valve. In refining the hysteresis performance, another problem was brought to light. This problem is the inability of the poppet to remain in alignment with the seat while the relief valve is functioning. To machine the parts of a relief valve with close enough tolerance to control the maximum misalignment is impractical and would cause the parts to bind, which destroys the ability of the valve to open and close at a specific pressure level.
Typical prior art solutions to this alignment problem allow the poppet, seat or both to freely float, as typified in U.S. Pat. Nos. 3,054,420, 3,583,431 and 3,621,875. Each of these last-mentioned designs require sliding metal-on-metal friction to align the poppet and seat, as for example in U.S. Pat. No. 3,621,875, seat 22 must slide on ring 21.
The ability of a relief valve to maintain a constant pressure level as the flow rate across the valve increases, has been handled by designers in a variety of methods. The momentum exchange concept of impacting and turning a high velocity stream against a surface of the poppet to increase the load against the spring to counteract the spring rate, is taught in the above-mentioned Williams U.S. Pat. No. 3,054,420.
Another method is the area gain method as described in the Diel U.S. Pat. No. 3,583,431, above-mentioned.
A still further method is the use of sized orifices to create pressure drops at various locations on the poppet to counteract the changing spring rate. Different valve designs use various combinations of the above-mentioned methods to achieve a flat pressure-flow curve.